The present invention represents an improvement on the encoders disclosed in an application of George C. Devol, Ser. No. 507,082 filed Sept. 18, 1974 now abandoned. That encoder includes a rotary selector that cooperates with a circular series of elements. A train of pulses is developed by successively testing the elements, limiting the number of pulses in a train in accordance with how many elements there are between an element selected by the selector and the start or the end of the series of elements. The pulses are entered into a counter that provides a code representing the shaft position.
The physical limitation on how close one element can be located to the adjacent elements of the series imposes a limitation on how many discrete positions can be encoded with a practical size of the circular series of elements. This depends on the form of element used (magnetic, capacitive, photosensitive, etc.) and the circuit used to recognize cooperation of the selector with the element. The broad proposition was recognized by George C. Devol that the number of discrete codes for a shaft position encoder can be increased by changing from one circular series of elements and a rotary selector, to two selectors coupled to the shaft whose position is to be encoded and coupled to each other by gearing such that the "fast" selector moves through its entire range while the "slow" selector advances from one element to the next.